Frequency-compensated television camera circuits



Dec. 26, 1950 J. F. WlGGlN 2,535,711

FREQUENCYCOMPENSATED TELEVISION CAMERA CIRCUITS Filed Dec. 8, 1948 4 VDEO OUTPUT AMPLIFIER-L- VIDEO AMPLIFIER Fig.2. 4001 R5 Cs 300- (0 Z I Q I 200- Ld F" 3 z g 3 IL! L E mo FREQUENCY M c.

Inventor": Joseph Fiwiggin,

His Attorney.

Patented Dec. 26, 1950 FREQUENCY-COMPENSATED TELEVISION CAMERA CIRCUITS Joseph F. Wiggin, Syracuse, N. Y., assignor to General Electric Company, a corporation, of

New York Application December s, 1948, Serial No. 64,183

My invention relates to television camera circuits and, more particularly, to frequency compensation of the load circuits associated with a television camera tube.

In a television camera pickup system, it is usually necessary to use a relatively high resistive load across the camera tube in order that a good signal to noise ratio may be achieved. Conventional camera tubes, such as iconoscopes, deliver very small signal currents which make a high resistance load necessary in order to obtain fairly large signal voltages, especially in the low frequency region. However, such a resistive load is accompanied with inevitable distributed shunt capacity which causes the response at high frequencies to be greatly reduced. In order to obtain good picture quality without distortion, it is necessary that the frequency response be substantially linear over the band of operating frequencies corresponding to the picture signals.

This band is, of course, approximately 5 megacycles wide in current television broadcasting practice. Without compensation, the effective load impedance Z0 decreases with frequency, approaching a value inversely proportional to the frequency, as shown by: I

herent distributed shunt capacitance across Ro. Hence, it can be seen that some form of compensation is necessary in order that the overall response may have a constant value over the operating range of frequencies.

One of the means that has been used. for compensation, and which is theoretically exact in its simplest form, comprises an impedance Zc, having inductance and resistance in series, connected as the plate load in an amplifier coupled to the camera output, in which the impedance is: Z,=R,(1 +1.01%) (2 where R0 is the load resistance and Co the in- 4 Claims. (Cl. 179171) making Lo and Re small so that their resonant frequency will lie far above the desired band of frequencies, this rise in response at the high frequencies can be made as low as desired. It

has the disadvantage, however, in that it results in reduced gain for the compensating stage. In fact at low frequencies the gain is often so low that a good signal-to-microphonic-noise ratio cannot be maintained without overloading the tube driving the compensating network.

It is an object of my invention to provide an improved camera tube amplifier overcoming the above difficulties.

A further object of my invention is to pro-- vide a television amplifier compensating circuit in which the frequency response is linear over a wide range of frequencies.

Another object of my invention, is to provide a camera tube amplifying system having a large gain and ideal frequency response over a large band of frequencies.

For additional objects and advantages, and forv a better understanding of my invention, attention is now directed to the following description and accompanying drawings, and also to the appended claims in which the features of the invention believed to be novel are particularly pointed out.

In the drawings:

Fig. 1 diagrammatically shows a portion of a1 television camera circuit containing an embodiment of my invention;

Fig. 2 is a schematic diagram of the compensating circuit of my invention;

Fig. 3 is a diagram of a similar circuit but without compensation; and

Fig. 4 is a graph illustrating certain frequency response characteristics of the system of my invention.

Referring now to Fig. 1, I have illustrated, in

simplified form, a camera tube I having an out-,- put load resistor 5. The output developed across the load resistor 5 is supplied to amplifier stages 2, 3 and 4. It is understood, however, that there may be more or less stages of amplification. I have shown four stages for illustration only. Inasmuch as the tube I and amplifier stages 2 and 4 are conventional, a detailed explanation of these parts will be omitted for simplicity.

The stage 3 comprises a conventional pentode amplifier 6 having an --anode I, cathode 8, con trol grid 9, screen grid 10, and suppressor grid II. The cathode 8 and suppressor grid II are connected directly to ground potential. The control grid 9 is connectedto the video output of the preceding stage 2. Anode operating potential is supplied from suitable source of unidirectional voltage, designated by the conventional symbol B+, through resistors I2 and I3. A capacitor 14 connects the common terminal of resistors l2 and I3 to ground. The screen grid I obtains suitable bias from the B-lsupply through a resistor l5 which is bypassed to ground by a capacitor IS. A coupling capacitor l'l connects the anode l to a compensating circuit I8 comprising inductance l9 and resistance 20 connected in series and shunted by resistance 21. The adjustment and function of this circuit will be discussed in greater detail below.

A capacitor 22 and resistor 23 couple the video output from the circuit I8 to the grid 25 of a second pentode amplifier 2'4. The anode 2,6 of. the pentode 24 is connected to the input of the final video stage 6. Operating bias for the grid 25 is obtained through resistances 23 and 2! connected to the junction of resistances 28 and 29. Resistance 29 is connected to ground and resistance 28 is connected to a source of. negative voltage; designated by the conventional symbol, 0-. Gain control can be obtained by varying the negative bias, such as by varying resistance 29.

While I have, for purposes of illustration. shown the compensating circuit in the stage; 3, it may be. in any of the other amplifier stages.

In the compensating. circuit :8, as described above, the resistor 2i loads the series circuit id, 29 so that its response actually drop below the ideal response at the higher frequencies but approximate the ideal linear curve over the entire band of picture signal frequencies will be better understood from Figs. 2, 3 and l and the following description.

Let the load resistance across the camera tube be: represented by Re and the inherent distributed shunt capacity to be compensated be represented by Co. Fig. 2 represents the compensating network 2 ii, of Fig. l, in which LC is the serie inductance of coil 59, Re is the series resistance of coil 59 and resistor 20 and R5 the shunt resistance 2i. The inherent distributed capacity-across network 58 is also represented by Cs. Fig. 3 represents the compensating circuit of the prior art, which is the same except for the omission of resistance R5.

"The general equation for impedance Zc of a compensating circuit, as shown in Fig. 2, is as follows:

. L. Rg= /2L 6'.,RC C.

Disregarding the second-order term which is very small in usual practice, this may be simplified for practical purposes to:

Assuming a is the ratio of the impedance of the compensating circuit to the ideal impedance for an overall linear frequency response and 0 is the phase angle of this ratio, then it can be shown that:

For the case without shunt Rs:

Consider a. numerical example based on ac-. tual television camera practice. The impedance can be compared with and without the. correcting resistor 21 as illustrated by the. curves in Fig. i. For example, an input circuit of 2 micromicroseconds time constant will be as-- surned, thus requiring a compensation circuit which also has a two micrornicrosecond' time constant. The maximum departure allowable from the ideal impedance at 5 megacycles is assumed to be 3%. A value of 30 micrornicrofarad's will be assumed for the value of C5 which is typical for interstage coupling circuit capacity. By using these assumptions, proper circuit values can be calculated with and without the shunt re-' sistance and the impedance functions can be produced showing the relative gains obtainable.

When Rs is not used, it can be shown that for 3% departure from ideal, a=.l65

lama-R. 01

the error in neglecting RC CF. compared 2' LcCsis only one part. in 20,000 for this example,

The impedance versus frequency characteristic is shown in Fig. 4 as the curve B. The desired or ideal response is shown by the curve I. Using the above values, the impedance versus frequency characteristic with R5 omitted is shown in curve C.

These values produce a drop in the ratio of the impedances below unity which is approximately proportional to the fourthpower of frequency and will be approximately 3% at the maximum frequency of the video band. The drop then continues so that at resonance of Le, Cs the ratio is about .7. A phase lag of 42 maximum is produced but this phase angle is very nearly proportional to the frequency and hence introduces a time delay which is approximately constant over the frequency range in a manner similar to a conventional compensated video amplifier stage.

By the use of the shunt resistor, it can be seen that an improvement in gain of about 8 times can be realized without sacrifice in the uniformity of response as is illustrated by the curves in Fig. 4.

While certain specific embodiments have been shown and described, it will, of course, be under stood that various modifications may be made without departing from the invention. The appended claims are, therefore, intended to cover any such modifications within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a television camera tube, a load impedance network for said tube having inherent shunt capacity, an output network elec trically coupled to said load network, and a compensating network connected across said load network, said compensating network having two parallel branches, one branch comprising inductance Lo and resistance in series and the other branch comprising resistance R. and shunt capacitance Cs, said load and compensating net-- works having substantially the same time constant and the shunt resistance R having a value substantially equal to whereby the frequency response is substantially linear over a range of video frequencies supplied to said output network.

2. In a television camera circuit, a source of video signals, a load circuit having resistance and shunt capacity connected across said source, and a compensating circuit connected across said load circuit, said compensating circuit having two parallel branches, one branch comprising an inductance Lo and resistance in series and the other branch comprising shunt resistance R and shunt capacity Cs, said load circuit and said compensating circuit having substantially the same time constant and the value of said shunt resistance B being proportioned to have a value substantially equal to whereby the frequency response is substantially linear over a range of video frequencies supplied to said load circuit.

3. In combination, a television picture tube, an output load impedance network for said tube having inherent shunt capacity, a plurality of amplifier stages for the output developed across impedance, and a compensating network connecting one of said amplifier stages to another of said amplifier stages, said compensating network comprising inductance Lo and resistance R in series shunted by a resistance and inherent shunt capacity C5, said two networks having substantially equal time constants and said shunt resistance being so proportioned to have a value substantially equal to whereby the over-all frequency response is substantially linear over a range of approximately 5 megacycles at the input to said other amplifier stage.

4. In combination, a television camera tube, a load impedance network for said tube having inherent shunt capacity, an output circuit connected across said load impedance, and a compensating network connected across said output circuit, said compensating network comprising inductance Lo and resistance in series shunted by a resistance and capacitance Cs, said two networks having substantially equal time constants and said shunt resistance having a value substantially equal to whereby the frequency response has a nearly constant value over an operating range of video frequencies supplied to said output circuit.

JOSEPH F. WIGGIN.

REFERENiDES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,931,664 Lavoie Oct. 24, 1933 2,299,875 Bedford Oct. 27, 1942 2,299,891 Fredendall Oct. 27, 1942 2,309,744 Bedford Feb. 2, 1943 

